He was distracted by Voldemort.

I agree with what the others said - his handwriting most likely changed over the years, at least enough not to be easily recognizable. My handwriting changed since I was a teenager as well.. it’s definitely an interesting point though.

Because his handwriting has

changed in over 20 years

That’s a good question... maybe it’s a mistake by writer... but i think others are right. It’s about changing in the time

I was also thinking maybe the handwriting he used when writing on the board was different, because the notes in the book were meant just for him, so it would be his actual handwriting. I used to teach (briefly but still) and I would always make sure whatever I’m writing on the board is 100% legible, unlike when I write stuff in my own notes, I can read my own handwriting of course but I don’t care if it’s clear or not for others .. so it’s quite different than the style of handwriting I would use when teaching. Most likely his just changed over time though ..

And also in order of phoenix every teacher use magic to fill the board which it is likely a habit. For example:

'‘Today we will be mixing a potion that often comes up at Ordinary Wizarding Level: the Draught of Peace, a potion to calm anxiety and soothe agitation. Be warned: if you are too heavy-handed with the ingredients you will put the drinker into a heavy and sometimes irreversible sleep, so you will need to pay close attention to what you are doing.’ On Harry’s left, Hermione sat up a little straighter, her expression one of utmost attention. ‘The ingredients and method –’ Snape flicked his wand ‘– are on the blackboard –’ (they appeared there) ‘– you will find everything you need –’ he flicked his wand again ‘– in the store cupboard –’ (the door of the said cupboard sprang open) ‘– you have an hour and a half … start.’'

Maybe magic is the reason of difference.

Oh yea I forgot about that, but it would still be written in his handwriting though, like pre-written ahead of time sort of maybe

As in, how he thought/wanted it to appear. Yeah.

Shapes writing might be a little bit different, when he was young!!!

Hahaha! Harry is clueless when it comes to his surroundings. He misses everything. If he would have just paid attention, we would have a much different series!